1. Field of the Invention
This invention relates to a recently proposed moving picture coding system for storage media which handles digital moving picture, and more particularly to a technique of decoding and reproducing moving picture for special reproduction mode such as reverse reproduction.
2. Description of the Related Art
Recently, various systems for compressing and expanding digital moving picture have been proposed in relation with development of multi-media technique. Digital moving picture compressed by those techniques are expanded (decoded or restored) by conducting interpolation on plural original frame pictures and predicting movement of picture to produce predictive picture, thereby compensating for deficiency in information quantity of original pictures. Particularly, a system proposed in MPEG (Moving Picture Coding Experts Group) which is an international standardization conference of moving picture coding for storage media (hereinafter referred to as "MPEG system") is standardized in consideration of special mode reproduction such as random access or reverse reproduction.
Generally, in moving picture, contents of successive frame pictures are analogous to each other. Taking this into consideration, the moving picture coding technique produces intermediate picture between original pictures by interpolation. The MPEG system also relates to the interpolation technique. In the following, description will be given by taking the MPEG system as representative of such compressing and expanding systems of digital moving picture.
The moving picture coding system for storage media utilizes random-accessible media such as CD-ROM as the storage media. Namely, compressed data of original pictures are recorded on such a storage media. A decoding device of the coded moving picture reads out the compressed data of original pictures from the storage media and carries out digital signal processing including interpolation for producing intermediate pictures between the original pictures. Then, the device stores the interpolated (predicted) pictures in a memory, and outputs the stored pictures in succession as moving picture. Data structure of MPEG system is of hierarchical structure, and includes video-sequence stage, GOP (Group Of Picture) stage which can be individually reproduced, picture stage for predicting picture information including luminance and color differences, slice stage of scanning order, macro-block stage of luminance and color difference blocks of right-left and up-down directions, block stage including 8-lines.times.8-pixels of luminance and color differences. The decoding device performs prediction of picture, from the compressed pictures read out from the storage medium, utilizing Motion Compensation (MC) and Discrete Cosine Transform (DCT) between frame pictures having periodical intervals, and produces predictive pictures. Thus, the coding device stores decoded pictures by GOP units in a memory device in normal order, and then outputs them.
In order to demonstrate concept of predictive pictures, prediction of picture will be described below. FIG. 1 is a diagram illustrating pictures including predictive pictures in bit-stream of MPEG system. FIG. 1 shows frame pictures, and each of the frames corresponds to a picture stage in the data structure. Picture represented by the reference I is I-picture (Intra-coded picture) which constitutes a single picture by itself. Picture represented by the reference P is P-picture (predictive-coded picture) which is a predictive picture produced by prediction utilizing I-picture and other P-picture of past in reproduction sequence. Picture represented by the reference B is B-picture (Bidirectionally predictive-coded picture) which is produced by prediction utilizing not only I-picture and P-picture of past in reproduction sequence but I-picture and P-picture of future. The pictures shown in FIG. 1 constitute GOP stage. The pictures shown in FIG. 1 are decoded and produced by predictions in the order of P1, P2, P3, . . . , P6. Namely, P-pictures are firstly produced using I-pictures and P-pictures by steps of P1 and P2, and then B-pictures are produced using the I-pictures and P-pictures by steps P3, P4, . . . , P6. In this manner, pictures in a GOP are produced. Then, the decoding device stores the pictures thus produced, and reads them out in normal order. Such a technique is disclosed in Japanese Patent Application Laid-Open No. 5-176694.
On the other hand, when reverse reproduction is needed, the pictures of a GOP are produced and stored in the same manner, and then reproduced in reversed order. However, in order to carry out reverse reproduction in this manner, the decoding device should have memory of large capacity, which is relatively expensive. Namely, the decoding device produces plural pictures from one I-picture to next I-picture by GOP units, and stores all of the pictures in the memory. Then, the device reads out and displays the stored pictures one by one in an order reverse to the normal reproduction order. Therefore, the device is required to store the predicted, non-compressed pictures for many frames. In order to achieve slow reverse reproduction (reverse reproduction slower than normal speed), frame memory having storage capacity of at least 15 frames is required. Since approximately 2M bits of storage capacity is necessary for storing picture of one frame, the device should have large memory having capacity of 30M bits (for 15 frames). In this view, it is impossible, in practice, to achieve reverse reproduction by device of reasonable cost. In addition, storage of all frame pictures in GOP requires long period of time. However, in the field of digital moving picture compressing and expanding technique in which it is expected that special reproduction such as random-accessing, high-speed reproduction or reverse reproduction is readily and rapidly carried out, it is not preferable that storage of pictures takes such a long time.